1. Field of the Invention
This invention relates to an electrical device including a battery pack and a battery management system that determines a state of charge percentage of the battery pack based, at least in part, on a pressure reading from a pressure sensor adjacent the battery pack, a terminal voltage reading from the battery pack, and a temperature reading from the battery pack. More particularly, the invention relates to bulk forces exerted by a lithium on cell during various operating conditions and their effects on state of charge.
2. Description of the Related Art
Vehicles are used to facilitate modern transportation. Different energy sources (e.g., hydrocarbon fuel, battery systems, capacitance systems, compressed air systems) can be used to generate the power needed to facilitate movement of the vehicle. Electric vehicles, also referred to as all-electric vehicles, include a battery system and utilize electric power for the entirety of their motive power. A plug in power source is needed for electric vehicles for charging.
Hybrid vehicles and plug-in hybrid electric vehicles include both an internal combustion engine and a battery system. The battery is capable of being charged from a plug-in power source. Additionally, the internal combustion engine can turn a generator, that supplies a current to an electric motor to move the vehicle.
Knowing the amount of energy that is left in the battery gives the vehicle operator an idea of how long the vehicle can be used before recharging must take place, State of Charge (SOC) is an estimation used to compare the current state of the battery to the battery at full charge. The preferred SOC reference should be the current capacity of the cell, as the cell capacity is reduced by age, temperature, and discharge effects.
In all-electric vehicles, the SOC is used to determine the distance a vehicle can travel. SOC should be an absolute value based on current capacity of the battery to reduce error when the battery ages. When the SOC has fallen to a threshold, the driver of the vehicle must recharge the vehicle, much like refilling the fuel tank in a car. In hybrid electric vehicles, SOC determines when the engine is to be switched on and off. When SOC has fallen to a threshold, the engine is turned on and provides power to the vehicle.
Knowledge of the SOC is particularly important for large lithium batteries, Lithium batteries are significantly reactive and need electronic battery management systems to keep the battery within a safe operating window. Lithium on batteries expand as they are charged and contract as they are discharged. These changes in volume and length are caused by the absorption and release processes of lithium ions in the active materials of the electrodes. The absorption of the lithium into the carbon material causes the material to expand. An understanding of the SOC allows for control of the volumetric change of the battery, decreasing premature wear or ageing processes in lithium-ion batteries.
Lithium intercalation and de-intercalation result in the volumetric changes in both electrodes of a lithium-ion battery cell. At the anode, carbon particles can swell by as much as 12% during lithium intercalation, and the resulting stress can be large [Ref. 1 ], Commercial battery packs involve numerous cells assembled to occupy a fixed space as shown in FIG. 1 and held in mild compression to resist changes in volume associated with lithium intercalation and de-intercalation. A small compression prevents de-lamination and associated deterioration of electronic conductivity of the electrodes. A large compression, however, can decrease the separator thickness and lead to degradation and power reduction due to the separator pore closing [Ref. 2 ].
The effect of expansion and the system's mechanical response on the cell performance and life [Refs. 2, 3, 4, 5] are under intense investigation with studies ranging from the micro-scale [Refs. 6, 1, 5], the particle level [Ref. 6], and multiple electrode layers [Refs. 7, 8]. While progress towards predicting the multi-scale phenomena is accelerating [Refs. 9, 10], the full prediction of a cell expansion and its implications to cell performance depends heavily on the boundary conditions associated with the cell construction and electrode tabbing and crimping. Moreover, the wide range of conditions with respect to C-rates and temperatures that automotive battery cells must operate make the physics-based modeling approach very challenging. (The C-rate specifies the speed a battery is charged or discharged. For example, at 1 C, the battery charges and discharges at a current that is par with the marked amp-hour (Ah) rating. At 0.5 C, the current is one half and the time is doubled, and at 2 C the current is two-times and the time is one half.) Finally, measuring and quantifying the internal stress or strain to tune or validate the multi-scale models requires complex instrumentation [Refs. 11, 12 ]. In contrast to the micro-scale, the macro-scale stress and strain responses are directly observable and measured with high accuracy [Refs. 13, 14, 15], thus could be used to develop phenomenological models inspired by the underlying physics.
A phenomenological model that mimics the evolution of bulk force/stress and that quantifies the contributions of state of charge dependent intercalation and thermal effects would be desirable. The model could then be used for regulating the power drawn to avoid damaging forces and stresses on the cell [Ref. 16]. Such a methodology would enable a power management scheme that is conscious of mechanical limits similarly to the electric (voltage) and thermal limits as in Ref. 17.
Thus, what is needed is an electrical device including a battery management system that more preciously determines a state of charge percentage of the battery pack, particularly for lithium ion batteries.